A target is traditionally characterized by its radar cross-section (RCS). The RCS of a radar target is defined from the power balance of the wave emitted toward the target and the power of the wave received by the radar. In far field and by approximating the waves to plane waves, the radar equation is written:
                              P          r                =                              P            e                    ⁢                      G            e                    ⁢                      1                          4              ⁢              π              ⁢                                                          ⁢                              d                2                                              ⁢          σ          ⁢                      1                          4              ⁢              π              ⁢                                                          ⁢                              d                2                                              ⁢                      G            r                    ⁢                                    λ              2                                      4              ⁢              π                                                          (        1        )            
where Pe and Pr are the wave powers transmitted and received by the radar, respectively, Ge and Gr are the antenna gains upon transmission and reception, d is the distance between the radar and the target, λ is the wavelength used by the radar. The coefficient σ is homogenous to a surface and only depends on the considered target, which is the RCS of the target.
In expression (1), it is assumed that the radar used to illuminate the target was the same as that serving to receive the diffracted wave, which is then referred to as a monostatic RCS. As a general rule, the monostatic RCS depends on the direction of the incident wave, the frequency f of the radar and the respective polarizations πe and πr with which the incident wave is transmitted and the received wave is analyzed. It is denoted SER(f,ω,θ,πe,πr), where (ω,θ) are the azimuth and roll angles of the radar in a referential related to the target. Each of the polarizations πe and πr can be horizontal or vertical, i.e. πe=H or V; πr=H or V.
The measurement of an RCS is done in an anechoic location, i.e. a location whereof the walls are covered with sound-absorbing materials, so as to prevent parasitic echoes. The target is positioned using a lowly-echogenic positioner, generally on a vertical column made from polystyrene that can be oriented around its eigenaxis. The measurement is done either using a single antenna, or using two separate antennas that are slightly angularly offset relative to each other. Depending on the case, one obtains the monostatic RCS or quasi-monostatic RCS values, for one or more azimuth angles. The transmission antenna must be chosen so as to generate as flat a wave as possible.
Characterizing an object by its RCS has a certain number of drawbacks.
First, it is very expensive to build an anechoic location for large objects. This problem is worse at low frequency, i.e. for wavelengths in the vicinity of the size of the object, i.e. traditionally in the vicinity of one meter to several tens of meters, where the effectiveness of the sound-absorbing materials is lower. The measurements done are generally affected by noises from various sources (parasitic echoes, instrumentation noise, etc.).
Also in a low-frequency system, the illumination antenna must be large so as to be able to generate approximately planar waves.
Lastly, generally only several RCS value measurements are done, typically according to several azimuth angles in an equatorial plane so that one only has a fairly summary directional characterization of the target.
The RCS measurement therefore assumes using significant material resources and does not, particularly in low frequency, make it possible to characterize the electromagnetic diffraction by an object satisfactorily.
Furthermore, in the monitoring or maintenance field, it is often sufficient to verify the compliance of the characteristics of an object with a predetermined template or to evaluate the evolution of these characteristics over time. The recourse to characterization by RCS can then prove needlessly complex relative to the goal sought.
The object of the present invention is to propose a method for electromagnetically characterizing an object that is easy to carry out, including in the low frequency field, and which makes it possible, however, to verify that the object has electromagnetic diffraction characteristics according to a template or reference values, in relevant directions.